For many years, window and door frames were made from aluminum extrusions, which of course were highly durable. However, there were some associated problems. First, aluminum is an excellent thermal conductor, and the ability of these frames to rapidly transfer heat from the interior of a building to the exterior presented a problem as concerns about energy conservation have increased. Further, when the ambient exterior temperature is cold, condensation tends to form on the interior side of the frame, and this moisture has a tendency to run onto and damage painted surfaces, carpeting and the like.
Accordingly, an alternative approach was developed, in which two aluminum parts serving as the interior and exterior portions of the frame are connected only by an insulating material with a very low coefficient of thermal conductivity. A conventional and common manufacturing process for making such an arrangement is to first produce a single aluminum extrusion having a pair of C-shaped portions which are spaced from each other and define respective pockets that open toward each other, and having a bridge portion extending horizontally from one C-shaped portion to the other. A liquid plastic material is then poured into a region between the C-shaped portions so that it fills the region between them and also fills the pockets, the bridge portion holding the liquid plastic in place until it has hardened. After the plastic has hardened, the bridge portion is mechanically ground away with a grinding wheel or the like, leaving two separate extrusion parts which do not directly contact each other but are rigidly interconnected by the hardened plastic. This conventional approach is very successful in solving the traditional problem discussed in the preceding paragraph. However, another problem unique to this approach has developed.
More specifically, because the aluminum frame parts and the plastic material have different coefficients of thermal expansion, the plastic and aluminum tend to shift in a lengthwise direction with respect to each other over time. In fact, the plastic frequently has a tendency to shrink in overall length as this occurs, which is commonly referred to as dry shrinkage. The result of dry shrinkage is gaps between the aluminum extrusions just beyond the ends of the plastic material, through which rain or other moisture can enter the structure of a wall or floor supporting the window or door, with resulting water damage. Attempts have been made to chemically alter the formulation of the plastic so that it bonds more securely to the surfaces of the aluminum and so that it has little or no tendency toward dry shrinkage in length. To date, however, it has not been possible to chemically formulate a plastic which bonds tightly to the surface of an aluminum extrusion, no matter how thoroughly the surface has been cleaned, or a plastic which is satisfactorily free of dry shrinkage in overall length.
Therefore, although research for a chemical solution continues, attempts have also been made to mechanically resolve the problem, for example by mechanically interconnecting the plastic material to the extrusions in a manner effectively preventing relative movement. Mechanical approaches have included use of cooperating tabs and recesses on the plastic material and extrusions, and sandblasting of extrusion surfaces to roughen them so the plastic can achieve a strong bond. However, cooperating tabs and recesses are not very practical or efficient from a mass production viewpoint, and sandblasting an extrusion surface does not necessarily produce sufficient roughness to facilitate a particularly strong bond to the plastic, and is messy and inefficient from a production viewpoint. Accordingly, one object of the present invention is to provide an improved method and apparatus for mechanically roughening a surface of an extrusion so that, after a plastic material has been applied thereto in liquid form and hardens, the mechanical roughness is sufficient to reliably resist significant lengthwise movement of the plastic relative to the extrusion surface.